Stainless steel sheets have beautiful surfaces and excellent corrosion resistance, and are thus widely used for building facing materials, etc. Particularly, austenitic stainless steel sheets have excellent ductility and excellent formability for press, and cause no ridging, and are thus widely used for the above applications.
On the other hand, ferritic stainless steel sheets are improved in formability by the progress of the technique of purifying steel, and the use for above applications instead of austenitic stainless steel sheets of SUS 304, SUS 316, etc. have recently been studied. This is because the properties of the ferritic stainless steel, for example, the advantages of a low thermal expansion coefficient, low sensitivity to stress corrosion cracking, and low cost due to the absence of expensive Ni, have been widely known.
However, in consideration of application to formed products, the ferritic stainless steel sheets have lower ductility than the austenitic stainless steel sheets, and thus cause problems in that unevenness referred to as “ridging” occurs in the surfaces of the formed products to deteriorate the beauty of the formed products, increasing a load of surface polishing. Therefore, in order to further extend the application of the ferritic stainless steel sheets, improvements in ductility and anti-ridging property are required.
For these requirements, ferritic stainless steel having excellent formability and comprising, by weight %, 0.03 to 0.08% of C, 0.01% or less of Ni, and 2×N % to 0.2% of Al is proposed in, for example, Japanese Unexamined Patent Publication No. 52-24913. In the technique disclosed in Japanese Unexamined Patent Publication No. 52-24913, the C and N contents are decreased, and the Al content is twice or more as much as the N content to make crystal grains fine, thereby improving the ductility, r value (Lankford value), and anti-ridging property.
Heat-resistant ferritic stainless steel having excellent formability for press is proposed in Japanese Unexamined Patent Publication No. 54-112319, in which the (C+N) content is 0.02 to 0.06%, and the Zr content is 0.2 to 0.6% and 10(C+N)±0.15% to improve the ductility and the r value.
A method of producing a ferritic stainless steel sheet having excellent formability is proposed in Japanese Unexamined Patent Publication No. 57-70223, in which a ferritic stainless steel slab containing 0.08 to 0.5% of sol. Al, and at least one of B, Ti, Nb, V, and Zr is hot-rolled, cold-rolled, and then finally annealed.
However, the techniques disclosed in Japanese Unexamined Patent Publication Nos. 52-24913, 54-112319, and 57-70223 are mainly aimed at improving the ductility and r value, and remain the following problems:
(1) The techniques are premised on low C and N contents, and thus inevitably increase the cost of steel making process.
(2) Since elements such as Al and Ti are added, the amounts of inclusions of steel are increased, thereby inevitably causing surface defects due to the inclusions.
(3) The formability is greatly improved, while the anti-ridging property is not sufficiently improved. Therefore, in a case in which working such as press forming or the like is performed, the surface beauty of a formed product deteriorates, and polishing is thus required for improving the beauty to increase a polishing load, thereby increasing the cost.
Also, ferritic stainless steel having excellent corrosion resistance is proposed in Japanese Unexamined Patent Publication No. 59-193250, which contains 0.02% or less of C, 0.03% or less of N, and 0.5 to 5.0% of V. In the ferritic stainless steel disclosed in Japanese Unexamined Patent Publication No. 59-193250, corrosion resistance, particularly resistance to stress corrosion cracking, is significantly improved by adding V. However, the ferritic stainless steel disclosed in Japanese Unexamined Patent Publication No. 59-193250 has a problem of formability for press because no consideration is given to the formability for press.
Furthermore, ferritic stainless steel is proposed in Japanese Unexamined Patent Publication No. 1-201445, in which the P, S and O contents are decreased, 0.07% or less of C, 0.2% or less of Al and 0.15% or less of N and contained, and the relation between the (C+N) amount and the CR amount is optimized to improve formability and corrosion resistance. In the technique disclosed in Japanese Unexamined Patent Publication No. 1-201445, at least one of 40 S % to 2.0% of Mo, 20 S % to 0.5% of Ti, 20 S % to 0.5% of Nb, 20 S % to 0.5% of V, 20 S % to 0.5% of Zr, and 0.010% or less of B is contained to decrease both the amounts of solute nitrogen and carbon without limiting the relation between the (C+N) amount and the Cr amount, improving formability and corrosion resistance. In the technique disclosed in Japanese Unexamined Patent Publication No. 1-201445, Al, or further Ti, Zr, or the like is added, causing to increase the amounts of inclusions in steel, thereby causing the problems of inevitably producing surface defects due to the inclusions. It also remained the problem not to sufficiently improve the anti-ridging property.
Also, ferritic stainless steel having excellent weather resistance and crevice corrosion resistance is proposed in Japanese Unexamined Patent Publication No. 7-34205, which contains 0.05% or less of C, 0.10% or less of N, 0.03% or less of S, 5 to 50 ppm of Ca, 0.5% or less of Al, and 0.04% to 0.20% of P. However, the ferritic stainless steel disclosed in Japanese Unexamined Patent Publication No. 7-34205 has a high P content, and contains large amounts of Ca and Al. Therefore, the corrosion resistance is improved, while the formability is not sufficiently improved, thereby causing the problem of inevitably producing surface defects due to an increase in the amounts of inclusions.
Furthermore, a method of producing a ferritic stainless steel sheet for a floppy disk center core is proposed in Japanese Unexamined Patent Publication No. 8-92652, which has excellent formability for press and high surface hardness. The ferritic stainless steel sheet disclosed in Japanese Unexamined Patent Publication No. 8-92652 is a ferritic stainless steel containing 0.01 to 0.10% of C, 0.01 to 0.10% of N, and 0.1 to 2.0% of Mn, and impurities P, S, Si, Al and Ni at controlled contents. However, the ferritic stainless steel sheet disclosed in Japanese Unexamined Patent Publication No. 8-92652 requires control of surface roughness by final cold rolling to complicate the process, and is demanded to be further improved because of the insufficient formability.
In order to improve the anti-ridging property, strong draft in hot rolling is effective, as disclosed in, for example, Japanese Unexamined Patent Publication No. 10-53817.
In this way, the above-described conventional techniques are impossible to produce a ferritic stainless steel sheet which satisfies both surface quality and formability at low cost.
The present invention has been achieved for solving the above problems, and an object of the present invention is to provide a ferritic stainless steel sheet having good formability, and excellent anti-ridging property and surface quality after forming.